This invention relates generally to eddy current inspection and more particularly to high speed eddy current calibration.
Eddy current inspection is a commonly used technique for nondestructively detecting discontinuities or flaws in the surface of items made of electrically conductive material, including many gas turbine engine components.
Eddy current inspection techniques are based on the principle of electromagnetic induction in which eddy currents are induced within the component under inspection by application of alternating magnetic fields. Known eddy current probes include absolute probes, which contain a single inductive coil, and differential probes, which have a drive coil and a sense coil. In the case of a differential probe, eddy currents are induced in the component under inspection when the probe is moved into proximity with the component by alternating magnetic fields created in the drive coil. The eddy currents produce a secondary magnetic field that is detected by the sense coil, which converts the secondary magnetic field into an electrical signal that may be recorded and/or displayed for analysis. As the eddy current probe is passed over the component, the presence of cracks and other discontinuities or deformations in the component will produce changes in the magnitude of the induced eddy current as compared to the magnitude of the induced eddy current in areas that do not have such anomalies. This results in corresponding variations in the magnitude of the signal output by the sense coil. Hence, the output signal, specifically the amplitude of the output signal variations, is an indication of the condition of the component. An eddy current machine operator may then detect and size flaws by monitoring and analyzing the output signals.
Eddy current probes are typically calibrated by using a reference or calibration standard having simulated flaws of known dimension formed therein. Typically, the calibration standard is a square plate having one or more notches of prescribed dimensions electrical discharge machined (EDM) therein to simulate flaws. Calibration of an eddy current probe is accomplished by performing a rectilinear scan of the EDM notch in the calibration standard. The output signals obtained from scanning the known notch are compared to output signals produced by the probe from an actual flaw or crack in a component under inspection to provide an indication of the severity of the flaw.
The relatively small size of the calibration standards (they are typically a 2.5 to 3 inch square) present a short distance to be scanned during the calibration. Because of probe acceleration and deceleration times, the short scan distance limits the scan speed that can be achieved during calibration. This in turn limits the probe scan speed that can be utilized during inspection. Furthermore, in a production environment, eddy current calibration is often performed many times per day on each system (typically before and after each type of part is inspected). Frequent calibrations are needed because of the calibration accuracy that is currently available.
Accordingly, there is a need for eddy current calibration methods and systems that can achieve better scan speeds and require less frequent calibrations.
The above-mentioned need is met by the present invention which provides method and system for calibrating an eddy current inspection system having an eddy current probe and a turntable that is rotatable about an axis. A calibration standard having a notch of known dimensions formed therein is mounted on the turntable for rotation therewith. The eddy current probe is positioned adjacent to the calibration standard, and the calibration standard is then rotated so that the probe scans the notch. By providing a rotating scan of the calibration standard, higher scan speeds and more accurate calibrations are achieved.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.